1. Field of the Invention
The present invention relates generally to the field of bicycle transmissions, and more particularly, relates to an improved automatic derailleur shifter wherein the gear ratio between the driving and driven gears can be automatically varied in response to the power requirements.
2. Description of the Prior Art
Bicycle transmissions wherein the gear ratio can be automatically varied through a contractible and expandable, large diameter sprocket wheel construction in response to the effort exerted upon the pedals by the cyclist are known. In these types of devices which have been developed for use with the so-called five speed and ten speed transmissions, the effective diameter of the driving, large diameter sprocket wheel assembly has been varied by providing a plurality of cooperating sprocket gears and by changing the radial positions of the sprocket gears. The determination of the radial positions of the sprocket gears is an automatic function of the input torque and the parts are so arranged that by increasing the torque, the effective diameter of the sprocket wheel components will be decreased against the bias of an operating spring or springs. Conversely, by decreasing the input torque applied at the pedals, the continuously acting spring or springs function to automatically increase the effective diameter of the sprocket gears. In this manner, the gear ratio between the driving gears and the driven gears has been effectively automatically varied.
In the variable large diameter sprocket wheel types of transmissions, it was the usual practice to disconnect and remove the rear derailleur together with its control cable and lever and to permanently place the drive chain about one of the rear small cluster gears. Accordingly, all changes in the respective gear ratios of the bicycle transmissions were accomplished by changing the effective diameter of the large driving gear. However, all of these types of prior automatic transmissions suffered from a common problem in that the increase or decrease in the effective diameter of the sprocket wheel assembly could not be smoothly made and the engagement or disengagement of the driving sprockets and the driving chain was always accompanied by an unwanted and uncontrollable clatter or shock, a condition which is definitely detrimental when operating any bicycle transmission, and is especially so under racing conditions. Additionally, most of the prior designs tended to be cumbersome, complicated and somewhat ungainly in appearance.
Other prior workers in the art have attempted to improve the manner of varying the gear ratio by applying special constructions at or about the cluster gears to function the derailleur for gear change purposes. Some of the prior art constructions included electrical changers, pneumatic changers and other mechanical devices which were intended to remotely operate the derailleur. So far as is known, most or all of the prior art gear changing devices required conscious effort on the part of the bicycle rider in order to effect the desired gear change.
The prior art automatic gear ratio changing constructions have tended to be quite complicated and cumbersome in design, thereby resulting in increased manufacturing costs, in increased maintenance costs, in increased weight and in a plurality of interacting components that were continually subject to breakdown or wear. Accordingly, the need remains to provide a simple, lightweight and automatically operating mechanism that is designed to easily change the gear ratio of a bicycle transmission in a smooth manner without conscious effort on the part of the rider.